The Feynman's Lectures on Physics are based on a famous course of
undergraduate lectures given at Caltech
by Professor Richard Phillips Feynman in the early 1960's.
What Dick Feynman had to say to undergraduates about various physical
units was considered too trivial by the editors and was not included in the published
version of these lectures.
We resurrect it here, from the audio record, as a tribute to Richard P. Feynman.

For those who want some proof that physicists are human, the proof is in the idiocy of all the different units which they use for measuring energy.The Character of Physical Law (1967) R.P. Feynman
[ 1964-11-12 video ]

Before I begin the lecture [on spacetime], I wish to apologize for something
that is not my responsibility: Physicists and scientists all over the world have
been measuring things in different units, and causing an enormous amount of
complexity. As a matter of fact, nearly a third of what you have to
learn1
consists of different ways of measuring the same thing, and I apologize for it.
It's like having money in francs, and pounds, and dollars... with the
advantage over money that the ratios don't change, as time goes on.

For example, in the measurement of energy, the unit we use here
is the joule (J), and a watt (W) is a joule per second.
But there are a lot of other systems to measure energy.
There are at least three different ones for engineers, which I have listed
here.2

The physicists do something else when they want to talk about the energy
of a single atom, instead of the energy of a gross amount of material.
The reason is, of course, that a single atom is such a small thing that to talk
about its energy in joules would be inconvenient.
But instead of taking a definite unit in the same system
(like 10-20 J),
they have unfortunately chosen, arbitrarily, a funny
unit called an electronvolt (eV), which is the energy needed to move an
electron through a potential difference of one volt, and that turns out to be
about 1.6 10-19 J.
I am sorry that we do that, but that's the way it is for the physicists.

The chemists also talk about the energy per atom.
Since they don't use the atoms individually but large blobs of them,
in cans and barrels, they've chosen a certain number of atoms as a unit.
This number of things is called a mole (mol), and it is
6.023 1023 objects.
The more precise definition, which is now correct
or soon3 will be,
is that one mole of carbon-12 atoms has a mass of exactly 12 grams.
A mole is just a certain number of things.
So, instead of giving the energy per atom, the chemists give the energy per mole.
It's good, therefore, to know how much energy is a mole of electronvolts.
In other words, if each atom had
one electronvolt of energy, a large number of atoms would have a
reasonable amount of joules, namely 96500 joules per mole.
Incidentally, a mole of electrons has a total charge of 96500 coulombs (C);
these numbers are equal for a reason you have to
figure out.4

Now, there is an additional unit that the physical chemists use,
the kilocalorie per mole (kcal/mol), and 23 of those is an electronvolt per atom.
[23 kcal = 96500 J]

Finally, unfortunately, you have another system for measuring masses.
The mass of an atom, from a chemist's point of view, is given by the mass of a mole
of these atoms.
For example, the mass of carbon-12 is called 12 "atomic mass
units" (u), because a mole of carbon-12 "weighs" 12 grams
(or rather "has 12 grams of mass").
One atomic mass unit represents one gram for every mole of objects,
one gram per mole.
We can measure that in electronvolts also.
"You can't measure mass in electron
volts!"5
Sure you can,
because of the relation E = mc2 ...
It is useful to know how much energy corresponds to the consumption
of one atomic mass unit of material:
That turns out to be about 931 million electronvolts (MeV).
Incidentally, the rest mass of a proton is 938 MeV,
while the rest mass of an electron corresponds to 0.511 MeV.
The number 938 differs from 931, because a proton has a mass of about 1.008 amu.

I am sorry about the confusion produced by all these systems of units.
I left out, obviously, a large number of different things.
For example, when measuring
luminous6 energy,
the lumen (lm) is used, which corresponds to about 1.5 mW of
power in the "most visible" light, around 5500 Å (ångströms).
It's all very annoying, but don't worry about it now.
When you need to measure light, just look up in a book what a lumen is.

That's an unfortunate fact that we measure things
in a whole series of different kinds of units.
This causes a lot of confusion.

It's too bad, but I have already apologized, and there is nothing else I can do...

Richard P. Feynman (1961)

Notes:1 Feynman may well be overstating the case,
but the opposing understatement is so widespread
that we decided to feature this statement as a subtitle for this web page...
2
Consistent "engineering" systems of mechanical units are listed below.
The units of energy and power advocated by Feynman for use at CalTech,
the joule (J) and the watt (W), are metric (SI) units.
The other systems are now obsolete, or are being deprecated:

Metric system (SI) for mechanical units (MKS = meter-kilogram-second).The
unit of force is the newton (N).
The unit of energy is the joule [J].

CGS system (centimeter-gram-second).The unit of force is the dyne (dyn).
The unit of energy is the erg [erg].

British [absolute] system (foot-pound-second).The
unit of force is the poundal [pdl].
The unit of energy is denoted ft-pdl.

Metric-technical system, or "gravitational MKS"
(meter-hyl-second).The
unit of force is the kilogram-force [kgf].
The unit of energy is denoted kgm.

British engineering system, or "English gravitational system"
(foot-slug-second).The
unit of force is the pound-force [lbf].
The unit of energy is denoted ft-lb.

3 The CGPM finally adopted this definition of the
mole in 1971.
It is based on the "unified" definition of the "atomic mass unit"
as 1/12 the mass of a Carbon-12 atom, which was
adopted at the 10th General Assembly of the
International Union of Pure and Applied Physics (Ottawa, 1960)
and subsequently by the
International Union of Pure and Applied Chemistry in 1961.
The unified unit (symbol "u") replaced two earlier competing definitions,
both using the symbol "amu", which were based on different aspects of oxygen:
The physical amu was defined to be 1/16 of the mass of the Oxygen-16 atom,
while the larger chemical amu was 1/16 of the average mass of an atom of natural
oxygen, with the isotopic composition found on Earth
(99.759% O-16, 0.037% O-17, 0.204% O-18).
The unified amu (u) is only 0.004% more than this chemical amu.
Thus, the chemists got to keep the value of their amu, while the physicists got their
wish in having the atomic mass unit defined in terms of a given nucleus,
not an isotopic mixture. The symbol for the unified amu is "u".
A chemical amu was 0.999958 u,
a physical amu was 0.999687 u. [See
sizes.com].
The unified atomic mass unit (u) is increasingly referred to as a
"dalton" (symbol Da). That name was adopted by the IUPAC in 1993, by the CIPM
in 2003, by the IUPAP in 2005 and by ISO in 2009.4 Moving a mole of electrons through a potential of
one volt involves an energy equal to a mole of electronvolts. Expressed in joules,
this equals one volt multiplied
by the charge of a mole of electrons, expressed in coulombs
(Faraday's constant).5 To remove this objection, the "Mev of mass" is
now best abbreviated Mev/c2.
Similar notations are used for other
mass units obtained from various energy units.6 In the spoken lecture, Feynman wrongly used the
qualifier "radiant",
instead of the term "luminous" which properly qualifies electromagnetic radiation measured
with the spectral sensitivity of the human eye under normal "photopic" conditions
(when the eye adapts to darkness, it has a different spectral response,
which is called "scotopic").
Luminous power --often called "luminous flux"-- is expressed in lumens,
whereas the corresponding radiant power would simply be measured in watts.
The qualifier "bolometric" may be
used for radiant power, to insist that radiant measurements are meant
to include all energies at all frequencies with the same weight,
which is not the case for luminous measurements.

Current
(CODATA-2010)
recommended values for the fundamentalconstants
and the conversion factors quoted by Richard Feynman:

Precision
is indicated by the estimated standard deviation (betweenparentheses),
expressed in units of the least significant digit shown.

In 1935, the thermochemical calorie was defined as exactly equal to 4.184 J.
No other conversion factor is acceptable for modern chemists (or nutrionists).

The fifth International Conference on the Properties of Steam
(London, July 1956)
adopted the most authoritative version of the British thermal unit
(Btu) by equating 1 Btu/lb to exactly 2326 J/kg.
Conversion factors derived from this equivalence are identified by the
abbreviation IT or IST (International [Steam] Tables).
When the avoirdupois pound was finally defined in metric terms as
exactly 0.45359237 kg (effective January 1, 1959),
the IT Btu became equal to exactly 1055.05585262 J
(namely, 2326 J multiplied into the ratio of the pound to the kilogram).

To this official Btu corresponds, unfortunately, a widespread (bogus) "IST kilocalorie"
obtained by equating a kilocalorie per kilogram to 1.8 times the
above ratio of 2326 J/kg (since 1.8°F is 1°C).
This makes an "IT calorie" exactly 4.1868 J.
However, this wasn't a reason to revise thermochemical data that had been published
for decades with the firm 1935 equivalence of 4.184 J to the calorie.
Most authors were wise enough to hold on to that established equivalence but the
bogus conversion factor of 4.1868 J/cal has
infected computers, handheld calculators
and... the above table (sort of).

Since 1979 (16th CGPM)
the candela is defined so that
the "Mechanical Equivalent of Light" is exactly
one watt per 683 lumens (at 540 THz).

The speed of light in a vacuum (c) is Einstein's constant
(the name was advocated by Kenneth Brecher of Boston University, in April 2000).
Because of the (fifth and final) definition of the meter adopted by the 17th CGPM in 1983,
Einstein's constant is now exactly equal to 299792458 m/s.
In 1948, the definition of the ampere by the 9th CGPM equated the
magnetic constantmo
(the permeability of the vacuum) to
4p 10-7 H/m
(one henry per meter is the same as one newton per square ampere ).
The second has been defined in absolute terms since the 13th CGPM,
in 1967.
(The previous astronomical definitions are obsolete.)
The final fundamental step in the construction of the SI system would be to
redefine the kilogram by giving yet another fundamental constant an exact value.
For this transition to happen seamlessly,
current precision
on the known value of such a constant should be about as good as the
precision of comparisons with the international prototype of the kilogram.
We are almost there:
A large device (2 stories high) called a watt-balance
compares a mechanical watt (proportional to a calibrated mass) to an electric watt,
known in
terms of the Planck constant.
The best watt-balances have already determined
Planck's constant
[namely
6.626 069 57(29) 10-34 J/Hz]
to an accuracy (1 s) of about 0.044 ppm,
which is within a factor of 2 from the best mass comparisons.
The end is in sight.

We are proud to have this page belong to the Feynman Webring, which connects
a number of fine pages which perpetuate the legacy of Richard Feynman in various ways.
Some are more controversial than others:

Sommerfeld's Fine-Structure Constant may be viewed as the only
free parameter in QED,
the relativistic quantum theory of the interactions between electrons and photons
(for which Feynman, Schwinger and Tomonaga shared the
1965 Nobel Prize).
In QED, the coupling constant's effective limit is simply the square root of
a, and Feynman was understandably annoyed that QED was
thus based on an unexplained numerical value.
He expressed the wish that a deeper understanding of Nature would
eventually explain that value and/or allow it to be expressed in terms of
known constants, like p or e.

Before and after
Feynman, others have tried to guess such a relation,
possibly hoping that it could be a clue to whatever more general theory may underly
our current understanding of the laws of Nature.
Around 1923,
Sir
Arthur Eddington (1882-1944) "proved" a to be 1/136,
in agreement with early rough estimates.
When subsequently faced with incompatible experimental data,
he amended the "proof" to show that the value had to be 1/137,
so that Punch magazine dubbed him Sir Arthur Adding-One.
See 137 by Charles C. Mann,
or look up the description by
Robert Munafo of the so-called
Eddington
number [the outrageously asserted total number of electrons in the Universe,
as the inverse of the fine structure constant multiplied by a power of two].

The two integers in Dr. Gilson's dubious formula
may be tuned to accurately reflect modern experimental data only up to a point:
The pair (137,25) was the best match for the midrange of the previous
(1986) CODATA value of a [namely 0.00729735308(33)]
and (137,29) is a good match for the
current one (CODATA 1998, as of 2002).

Interestingly, Gilson quotes Michael Wales,
who had argued that the cube of a
should be the reciprocal of some integer (namely 2573380).
The 1986 CODATA value of a placed Wales' number at
2573379.99(35), which encouraged the conjecture.
However, the more precise 1998 CODATA update gives 2573380.571(29),
which does not stand any reasonable chance of being an integer!
Even more so with
2573380.5325(25) the value derived from CODATA 2010.

Likewise, for the pair (137,x) to match the value 137.036999076(44)
with Gibson's formula, the parameter x ought to be 28.645(23). Not an integer!

Gilson's formula and its justifications are pseudoscientific.
Gilson is [at best] guilty of wishful thinking when he
presents his formula as a generally accepted fait accompli.
This is simply not so. You've been warned;
you may http://www.fine-structure-constant.org/page5A.html.